An important refractive element of the eye, the cornea is the multi-layered, transparent, avascular, outermost part of the eye globe. For a human to see well, all layers of the cornea must remain transparent. Any cloudy or opaque area on a layer of the cornea will interfere with the proper refraction of light. The successive layers comprising the cornea, from the ocular surface inward, include the epithelium, Bowman's Layer, stroma, Descemet's membrane, and endothelium.
The human corneal endothelium, a single layer of cells lining the posterior surface of the cornea and facing the anterior chamber, plays a pivotal role in regulating corneal stromal hydration and hence, transparency. The human corneal endothelium has a critical fluid extraction or pumping function that is needed to maintain the transparency of the cornea. In a healthy eye, ocular fluid passes slowly from the interior to the stroma; and excess water is pumped from the stroma into the anterior chamber of the eye by the corneal endothelium. Further, it is critical that the rates of fluid moving into and out of the cornea are maintained in balance. If the pumping function of endothelial cells is diminished, the stroma would swell, and the regular pattern of the stroma's collagen matrix would be damaged by the excess water. This would result in the stroma becoming hazy, and eventually opaque.